I recall that Star Trek did, in it's later incarnations attempt to address the interbreeding of alien species. Vulcan-Human and Klingon-Human hybrids required some in-utero genetic manipulation to be viable.

As for Dragonball Z, there are far more puzzling scientific questions there than interbreeding, but I won't go into them. I have noticed that Human-Saiyans seem to exhibit some kind of hybrid vigour. They appear to be more powerful than pure Saiyans.

In Star Trek, cross-species breeding makes *some* degree of sense, since it is established in Next Generation that they all come from the same basic original "seed", as created by an ancient alien race billions of years earlier. Hence why so many humanoids of fairly analagous forms.